In a power supply device including an assembled battery configured of a plurality of battery cells, in order to prevent battery cells from being in abnormal states of over charge, over discharge, or the like, a state monitoring portion always monitors voltage, temperature, or the like of each of the secondary battery cells. Especially, in a case of using the assembled battery in which the plurality of the battery cells are connected in series, an voltage detector as the state monitoring portion for detecting the voltage of each of the battery cells, is provided. The detected voltage of each of the battery cells is used for controls of charge and discharge.
The voltage detector obtains an terminal electrical potential of each of the battery cells constituting the assembled battery, and detects the voltage of each of the battery cells through differential amplifiers or analog/digital converters. At this time, depending on a relation of frequency input signal inputted into the analog/digital converter and sampling frequency of the analog/digital converter, the accuracy of the voltage detection may decrease. Concretely, it is necessary that the maximum frequency of an analog signal inputted into the analog/digital converter does not exceed the half value (Nyquist frequency) of the sampling frequency. When the analog signal includes the frequency component which the Nyquist frequency, the input signal overlaps. In a case where the input signal overlaps, the output signal of the analog/digital converter becomes the digital signal including unnecessary signal information (aliasing error), the accuracy of the digital signal decreases.
As a method which resolves this problem, a power supply device which has a filter circuit between each of the battery cells and the analog/digital converter, has been known (Patent Literature 1). The filter circuit of Patent Literature 1 is a low pass filter, and when the analog signal corresponding to a potential of each of the battery cells is inputted into the filter circuit, the filter circuit attenuates the signal of the frequency equal to or more than the cutoff frequency corresponding to each of the analog signals, and outputs such a signal to analog/digital converter. By this configuration, the maximum frequency of the analog signal inputted into the analog/digital converter, can be decreased, and an occurrence of the aliasing error is prevented.
In the power supply device of Patent Literature 1, the capacities of the capacitors or the resistance values of the resistors are respectively set at most appropriate values, and then the frequency response of each of the outputs in the filter circuits can be equalized, while a resistor connected between the battery modules is common. Thus, it is necessary that the filter circuit has the same frequency response in each of the outputs in order to accurately detect the voltages.